Nonthermal electron-positron pair production and the 'universal' X-ray spectrum of active galactic nuclei

The process of resonant high-energy electron–positron pair production by an ultrarelativistic electron colliding with the field of an X-ray pulsar is theoretically investigated. Resonant kinematics of the process is studied in detail. Under the resonance condition, the intermediate virtual photon in the X-ray pulsar field becomes a real particle. As a result, the initial process of the second order in the fine structure constant effectively reduces into two successive processes of the first order: X-ray-stimulated Compton effect and X-ray-stimulated Breit–Wheeler process. For a high-energy initial electron all the final ultrarelativistic particles propagate in a narrow cone along the direction of the initial electron momentum. The presence of threshold energy for the initial electron which is of order of 100 MeV for 1-KeV-frequency field is shown. At the same time, the energy spectrum of the final particles (two electrons and a positron) highly depends on their exit angles and on the initial electron energy. This result significantly distinguishes the resonant process from the non-resonant one. It is shown that the resonant differential probability significantly exceeds the non-resonant one.

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Rapid X-ray variability in active galactic nuclei

Symposium - International Astronomical Union ◽

10.1017/s0074180900152842 ◽

1986 ◽

Vol 119 ◽

pp. 269-271 ◽

Cited By ~ 1

Author(s):

P. Barr ◽

R. Mushotzky ◽

P. Giommi ◽

J. Clavel ◽

W. Wamsteker

Keyword(s):

Active Galactic Nuclei ◽

Galactic Nuclei ◽

Seyfert Galaxies ◽

Low Earth Orbit ◽

Earth Orbit ◽

Rapid Variability ◽

X Ray ◽

Electron Positron ◽

Eddington Limit

SummaryRecent EXOSAT observations of active galactic nuclei are presented. Unlike earlier X-ray satellites (all of which flew in low earth orbit), the deep orbit of EXOSAT allows long continuous observations of celestial X-ray sources, uninterrupted by earth occultation etc. We present the results of EXOSAT observations of several AGN which have been seen to vary rapidly (timescale 0.2–6 hours). We also consider the implications of rapid variability in AGN. For Seyfert galaxies and quasars, we find a highly significant correlation between the timescale of variability and their X-ray luminosity. They are not, howwever, bounded either by the (classical) Eddington limit nor by efficiency arguments. We sugest, rather, that the emitting plasma is dominated by electron-positron pairs.

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Physical Processes in Active Galactic Nuclei

International Astronomical Union Colloquium ◽

10.1017/s0252921100086152 ◽

1986 ◽

Vol 89 ◽

pp. 324-345

Author(s):

Roland Svensson

Keyword(s):

Pair Production ◽

Active Galactic Nuclei ◽

Gamma Ray ◽

Rest Mass ◽

Galactic Nuclei ◽

Nonthermal Plasma ◽

Lorentz Factor ◽

Analytical Models ◽

Potential Wells ◽

Electron Positron

AbstractActive galactic nuclei (AGNs) emit continuum radiation evenly spread over up to ten decades in frequency from the radio into the gamma-ray range. Plausible emission mechanisms and their characteristics are reviewed. In the deep potential wells around black holes the mean energy per proton can reach 100 MeV. Part or all of this energy may be channeled to all electrons equally (thermal plasma) or, preferentially, into only a small fraction of the electrons (nonthermal plasma). In the former case thermal Comptonization of soft photons may be the dominant emission mechanism, while in the latter case the synchrotron and the inverse Compton scattering process (synchro-self-Compton) are likely to dominate.When the compactness parameter L (hν≈mc2 )/R. (power L, radius R) exceeds about 1030 ergs cm−1s−1 or L>Lc ≡ 1030R ergs s−1, then electron-positron pair production takes place due to photon-photon interactions causing the source to shroud itself with an electron-positron atmosphere. The efficiency of pair cascades in converting injected energy into electron-positron rest mass can reach levels of about 10% in static pair atmospheres. The emerging radiation is strongly modified by the pair atmosphere causing the spectrum to soften and to have characteristic breaks.For emission coming from a region near the Schwarzschild radius, L>10-3LEdd is sufficient to cause prolific pair production. Radiation pressure then drives a mildly relativistic pair wind with Compton drag limiting the Lorentz factor to be less then 10. The pair rest mass power is at most of the order of Lc.Most results so far on static pair atmospheres and pair winds are either qualitative or based on simple analytical models. Needed numerical treatments of both time dependent and steady radiative transfer of both the continuum and the annihilation line radiation in mildly relativistic flows are relevant not only for AGNs but also for gamma ray bursts and galactic black hole sources.

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